Do Lists support `allow_None`?

The [docs on Lists](https://param.holoviz.org/user_guide/Parameter_Types.html#lists) say that:

> Because List parameters already have an empty value ([]), they do not support `allow_None`.

However that appears to be not quite true? Is that a documentation issue?

```python
import param

class P(param.Parameterized):
l = param.List(allow_None=True)

p = P()

print(p.l, p.param.l.allow_None)
# [] True

p.l = None
assert p.l is None
```